Glutathione S-transferase enzymes (GST enzymes) are a superfamily of enzymes that play an important role in the metabolism of a variety of organic compounds, including chemotherapeutic drugs, carcinogens, environmental pollutants and other harmful foreign compounds. GST enzymes have been reported to be present in a wide variety of species including bacteria, yeast, plants, nematodes, insects, birds, fish and mammals.
GST enzymes catalyze the conjugation of glutathione to a variety of different organic compounds. The glutathione thiolate anion serves as a nucleophile in the enzyme catalyzed reaction to attack electrophilic carbon, nitrogen, sulfur or oxygen atoms of a substrate molecule. See Beckett, et al. (1993) Adv. Clin. Chem. 30:281-380 for a detailed description of specific GST-catalyzed reactions.
There are a large number of GST isoenzymes in all mammalian species. These isoenzymes are encoded by four different families of GST genes, which have been designated alpha, mu, pi and theta. The GST isoenzymes from all four families are cytosolic enzymes and are dimers with a submit molecular weight of about 26 kDa. There are multiple isoenzyme forms in both the alpha and mu GST families, and these enzymes are located primarily in the liver. The pi family of GST isoenzymes are found in the liver as well as in a large number of other tissues. Thus far, only one or two isoenzymes in the pi family have been identified. The theta family of GST enzymes has only recently been isolated from liver and it is unknown how many isoenzymes exist in this family. In addition, a microsomal GST enzyme has recently been identified that is apparently structurally unrelated to the above described cytosolic enzymes. See Beckett, et al., supra for a more detailed description of GST isoenzymes.
Many cancers treated with chemotherapeutic drugs develop resistance to the particular chemotherapeutic drugs used in treatment. Furthermore, overexpression of GST enzymes is an important cellular mechanism involved in the development of drug resistance by tumor cells. Isoenzymes from both the alpha and pi GST families play a role in the development of resistance to chemotherapeutic drugs. For example, overexpression of alpha GST isoenzymes has been implicated in drug resistance to alkylating agents such as chlorambucil and cyclophosphamide. In addition, the pi GST isoenzymes have been shown to be involved in the development of drug resistance to a variety of other chemotherapeutic agents such as adriamycin, vinblastine, actinomycin D and colchicine.
Herbicide resistance in weeds has emerged as a significant problem in agriculture. Furthermore, overexpression of GST enzymes in plant cells is an important mechanism for development of herbicide resistance. In particular, overexpression of GST enzymes appears to be involved in the development of resistance to chloracetanalides, thiocarbamates, and to triazines such as atrazine.
As described above, GST isoenzymes have medical importance due to their role in mediating drug resistance in cancer patients. In addition, the measurement of GST isoenzymes in vitro also has importance in diagnostic medicine. For example, the measurement of the pi isoenzyme of GST in tissue specimens is useful in pathology for the detection and diagnosis of a variety of different tumors. In addition, measurement of the alpha form of GST in blood is useful for the detection and monitoring of a variety of different forms of liver disease. (See Beckett, et al., supra for a detailed description of the clinical applications of GST isoenzyme measurements.
There is a need for selective inhibitors of GST isoenzymes for treatment of drug resistance in cancer patients. There is also a need for GST isoenzyme inhibitors to be used with herbicides to prevent herbicide resistance in plants. In addition, there is a need for selective inhibitors of GST isoenzymes to be used in the in vitro diagnostic testing for the measurement of specific forms of GST isoenzymes. These and other needs are addressed by the present invention.